Trondheim's execution ground in ancient times. Benedictine monks built a monastery on the island very early in the 11th century, probably one of the first two monastries in Scandinavia. In 1658 it was converted into a prison and fortress, later becoming a customs house. Its most famous inmate was the Danish Count Peder Griffenfeld, who spent 18 years as a prisoner there. Today "The Monks' Island" is a popular recreational area, with a nice beach and a restaurant. Open from May - September.